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Source Category | 1990 | 1997 | 1998 | 1999 | 2000 | 2001 | 2002 | 2003 |
---|---|---|---|---|---|---|---|---|
Agricultural Soil Management | 253.0 | 252.0 | 267.7 | 243.4 | 263.9 | 257.1 | 252.6 | 253.5 |
Mobile Sources | 43.7 | 55.2 | 55.3 | 54.6 | 53.2 | 49.0 | 45.6 | 42.1 |
Manure Management | 16.3 | 17.3 | 17.4 | 17.4 | 17.8 | 18.0 | 17.9 | 17.5 |
Human Sewage | 13.0 | 14.7 | 15.0 | 15.4 | 15.6 | 15.6 | 15.7 | 15.9 |
Nitric Acid | 17.8 | 21.2 | 20.9 | 20.1 | 19.6 | 15.9 | 17.2 | 15.8 |
Stationary Sources | 12.3 | 13.5 | 13.4 | 13.5 | 14.0 | 13.5 | 13.5 | 13.8 |
Remaining Settlements | 5.5 | 6.1 | 6.1 | 6.2 | 6.0 | 5.8 | 6.0 | 6.0 |
Adipic Acid | 15.2 | 10.3 | 6.0 | 5.5 | 6.0 | 4.9 | 5.9 | 6.0 |
N2O Product Usage | 4.3 | 4.8 | 4.8 | 4.8 | 4.8 | 4.8 | 4.8 | 4.8 |
Waste Combustion | 0.4 | 0.4 | 0.3 | 0.3 | 0.4 | 0.4 | 0.5 | 0.5 |
Agricultural Residue Burning | 0.4 | 0.4 | 0.5 | 0.4 | 0.5 | 0.5 | 0.4 | 0.4 |
Remaining Forest Land | 0.1 | 0.3 | 0.4 | 0.5 | 0.4 | 0.4 | 0.4 | 0.4 |
Total for U.S. | 382.0 | 396.2 | 407.8 | 382.1 | 402.2 | 385.9 | 380.5 | 376.7 |
Source: US Emissions Inventory 2005: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003
The principal human-related sources of N2O are described below. For each source, a link is provided to the report entitled "US Emissions Inventory 2006: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004," prepared by EPA, which provides detailed information on the characterization and quantity of national emissions from each source. This report, hereafter referred to as the "U.S. inventory report," provides the latest descriptions and emissions associated with each source category and is part of the United States' official submittal to the United Nations Framework Convention on Climate Change (UNFCCC). The U.S. inventory report also describes the procedures used to quantify national emissions, as well as a description of trends in emissions since 1990.
Agricultural
soil management. N2O is produced naturally in soils
through the microbial processes of denitrification and nitrification.
These natural emissions of N2O can be increased by a variety
of agricultural practices and activities, including the use of synthetic
and organic fertilizers, production of nitrogen-fixing crops, cultivation
of high organic content soils, and the application of livestock manure
to croplands and pasture. All of these practices directly add additional
nitrogen to soils, which can then be converted to N2O. Indirect
additions of nitrogen to soils can also result in N2O emissions.
Indirect additions include those process by which applied fertilizer or
manure nitrogen volatilizes into ammonia and oxides of nitrogen and then
is ultimately re-deposited onto the soil in the form of particulate ammonium,
nitric acid, and oxides of nitrogen. Surface run-off and leaching of applied
nitrogen into ground water and surface waters can also result in indirect
additions of nitrogen to the soil. The U.S.
inventory report provides a detailed description on N2O
emissions from agricultural soil management and how they are estimated
(see the Chapter entitled "Agriculture").
Mobile
and stationary sources of fossil fuel combustion. N2O
is a product of the reaction that occurs between nitrogen and oxygen during
fossil fuel combustion. The volume emitted varies with the fuel type,
technology, or pollution control device used, as well as maintenance and
operating practices. For example, catalytic converters can promote the
formation of N2O, although the latest technical modifications
to converters are addressing this problem. The U.S.
inventory report provides a detailed description on N2O
emissions from fuel combustion sources and how they are estimated (see
the chapter entitled "Energy").
Nitric acid production. Nitric acid is an inorganic compound used primarily as a feedstock for synthetic commercial fertilizer. It is also a major component in the production of adipic acid and explosives. Virtually all of the nitric acid produced in the United States is manufactured by the catalytic oxidation of ammonia in which N2O is formed as a by-product and is released from reactor vents into the atmosphere. The U.S.
inventory report provides a detailed description on N2O emissions from nitric acid production and how they are estimated (see the Chapter entitled "Industrial Processes").
Livestock manure management. Nitrous oxide is produced as part of the nitrogen cycle through the nitrification and denitrification of the organic nitrogen in livestock manure and urine. The production of N2O from livestock manure depends on the composition of the manure and urine, the type of bacteria involved in the process, and the amount of oxygen and liquid in the manure system. Nitrous oxide emissions are most likely to occur in dry manure handling systems that have aerobic (in the presence of oxygen) conditions, but that also contain pockets of anaerobic (in the absence of oxygen) conditions due to saturation. It should be noted that emissions from livestock manure and urine deposited on pasture, range, or paddock lands, as well as emissions from manure and urine that is spread onto fields, are accounted for under the source category of “Agricultural Soil Management”. The U.S. inventory report provides a detailed description on N2O emissions from livestock manure management and how they are estimated (see the Chapter entitled “Agriculture”).
Human sewage. Domestic human sewage is usually mixed with other household wastewater, which includes shower drains, sink drains, washing machine effluent, etc. and transported by a collection system to either an on-site (e.g., a septic system) or centralized wastewater treatment plant. Nitrous oxide (N2O) may be generated during both nitrification and denitrification of the nitrogen present, usually in the form of urea, ammonia, and proteins. These compounds are converted to nitrate via nitrification, an aerobic (in the presence of oxygen) process converting ammonia-nitrogen into nitrate (NO3). Denitrification occurs under anaerobic conditions (in the absence of oxygen), and involves the biological conversion of nitrate into dinitrogen gas (N2). N2O can be an intermediate product of both these processes. The U.S. inventory report provides a detailed description on N2O emissions from human sewage and how they are estimated (see the Chapter entitled “Waste”).
Adipic acid production. Although only responsible for about 1% of the total nitrous oxide emissions in the U.S., adipic acid production is an important category from an individual plant perspective and because of the efforts that have been made to reduce emissions from those plants. N2O is generated as a by-product during the production of adipic acid which is used in the production of nylon and as a flavor enhancer for some foods. This white crystalline solid is used in the manufacture of synthetic fibers, coatings, plastics, urethane foams, elastomers, and synthetic lubricants. The U.S. inventory report provides a detailed description on N2O emissions from adipic acid production and how they are estimated (see the chapter entitled "Industrial Processes").
Natural emissions of N2O primarily result from bacterial breakdown
of nitrogen in soils and in the earth's oceans. Globally, tropical soils
(primarily wet forest soils, but also savannas) are estimated to produce
6.3 Tg of N2O annually and oceans are thought to add around
4.7 Tg of N2O annually to the atmosphere (IPCC,
2001(c) ).
Together, these two sources account for over 70% of the natural sources.
Similar microbial processes in temperate region soils produce smaller
quantities of N2O. In some ocean areas, large areas of surface
water can become oxygen depleted, allowing active denitrification in open
water. Large amounts of oceanic nitrous oxide can also arise from denitrification
in marine sediments, particularly in nutrient rich areas such as those
of estuaries.
It is important in studies of N2O emissions to account for the various interactions between natural processes and human influences in the nitrogen cycle, since human impacts can significantly enhance the natural processes that lead to N2O formation. For example, the nitrogen nutrient loading in water bodies due to fertilization and run-off to streams can enhance N2O emissions from these natural sources. Human-related ammonia emissions have also been shown to cause N2O emissions in the atmosphere through ammonia oxidation.
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